Known as a vehicle for high lift work is the one comprising a turntable mounted on a body, a telescopic boom axially supported by the turntable and adapted to perform operations such as extension/retraction and derricking, and a platform attached to the distal end of the telescopic boom. At the time of operation, this vehicle can derrick and extend the telescopic boom such as to move the platform to a desirable high working position, thus allowing workers to work there.
An example of the telescopic boom for such a vehicle for high lift work, as shown in FIG. 19, is constituted, in a telescopic fashion, by a proximal boom 909a mounted to the body (not shown) of the vehicle so as to be able to derrick, an intermediate boom 909b inserted into the proximal boom 909a so as to be able to extend and retract therein, and a distal boom 909c inserted into the intermediate boom 909b so as to be able to extend and retract therein. Thus constituted telescopic boom 909 incorporates therein a telescopic cylinder 921 which drives the intermediate boom 909b so as to make it extend or retract relative to the proximal boom 909a. The distal end portion of the cylinder rod 921b of the telescopic cylinder 921 is attached to the inside of the proximal end portion of the proximal boom 909a; whereas a cylinder tube 921a is attached, by way of a stationary shaft 923 extending laterally therefrom, to the inside of the proximal end portion of the intermediate boom 909b.
At positions separated from the cylinder tube 921a by a predetermined distance, the stationary shaft 923 is provided with rotatable retraction sheaves 925, each of which is wound with a retraction wire 935, whose one end is fixedly attached to the inside of the proximal end portion of the distal boom 909c, whereas the other end is fixedly attached to the inside of the distal end portion of the proximal boom 909a. The distal end of the cylinder tube 921a is provided with two rotatable extension sheaves 927, which are located on the inner side than the retraction sheaves 925. Each extension sheave 927 is wound with an extension wire 937, whose one end is fixedly attached to the inside of the proximal end portion of the proximal boom 909a, whereas the other end is fixedly attached to the inside of the proximal end portion of the distal boom 909c.
Here, since one end of the retraction wire wound about each retraction sheave is connected to the distal boom, the one end extends obliquely with respect to the retraction sheave. Therefore, the thrust load caused by the retraction wires would act on the retraction sheaves, whereby there is a possibility of the retraction wires shortening their lives. A similar problem would also occur in the extension wires.
Also, the extension sheaves are disposed on the inner side than the retraction sheaves, and the extension wires wound about the respective extension sheaves extend over the upper side of the telescopic cylinder, such that one end of each extension wire is fixedly attached to the inside of the proximal end portion of the proximal boom. Therefore, when assembling the telescopic boom, the retraction wires and extension wires must temporarily be fixed separately from each other while being assembled, whereby a large amount of labor is necessary for the assembling operation of the telescopic boom, and it is difficult to adjust tensions of retraction wires and extension wires.
Further, in order to attain safety in operations by securing electric insulation between the platform and the body of the vehicle, the distal boom is formed with an insulating member. Since the extracting/retracting operation is effected such that the extension/retraction of the intermediate boom relative to the proximal boom and the extension/retraction of the distal boom relative to the intermediate boom are simultaneously performed with the same ratio, however, the telescopic boom as a whole must be elongated in order to secure an insulating distance (the amount of extension of the distal boom) which is required for the safe operation. As a consequence, there has been a problem that, depending on the installation conditions of the telescopic boom and vehicle, there is a possibility of workers on the platform receiving an electric shock when touching an electric wire or the like before securing the required insulating distance.
The distal end of the distal boom 909c is provided with a not-illustrated platform, which is equipped with a winch operable in response to a hydraulic pressure supplied thereto, an operating apparatus sending out a control signal to the vehicle in response to an electric power supplied thereto, and the like. Therefore, hydraulic hoses and electric cables (hereinafter referred to as "hose and the like") for connecting the body of the vehicle and the platform to each other are disposed within the telescopic boom 909. As shown in FIG. 20, the hose and the like 965, with their one end being held on the upper face of the proximal end portion of the proximal boom 909a, extend toward the distal end of the telescopic boom 909 along the upper face of the proximal boom 909a. The hose and the like 965 are wound about a sheave 961 attached to the distal end portion of the distal boom 909a so as to turn around toward the proximal end, pass through the gap between the proximal boom 909a and the intermediate boom 909b so as to extend to the proximal end portion of the intermediate boom 909b, and are wound about a hose sheave 963 attached to the proximal end portion of the intermediate boom 909b so as to turn around toward the distal end again. The distal end portion of the hose and the like 965 passes through the distal boom 909c so as to extend to the distal end of the latter. As being thus routed, the hose and the like 965 can extend or retract together with the extending/retracting operation of the telescopic boom 909.
Here, the hose sheave necessitates a hose separator for winding the hose and the like appropriately about the hose sheave such that they are separated from each other without becoming entangled, a stopper for keeping the hose and the like from dropping out of the hose sheave, and the like. Also, along with the extending/retracting operation of the telescopic boom, a repetitive motion of tension or flexure acts on the hose and the like, whereby there may be cases where the hose and the like repeatedly come into contact with the hose separator or the like, thus being worn out or damaged. Also, when an excess tension is applied to an electric cable, there is a possibility of its inner strands breaking. Further, there is a problem that, when the extension wire wound about an extension sheave is broken or drops out thereof, there is a possibility of the distal boom moving down toward the proximal end of the telescopic boom.